1. Field of the Invention
The invention concerns a process that uses an ion exchanger resin to recover the sodium contained in industrial alkaline waste and, in particular, in the "red mud" and sodium effluents coming from the manufacture of aluminum hydroxide according to the Bayer process.
2. Discussion of the Background
The Bayer process that is widely described in specialized scientific literature is the essential method for producing the aluminum to be transformed into metal aluminum by igneous electrolysis or to be used as industrial alumina.
According to this process, bauxite ore is digested by a hot concentrated sodium hydroxide aqueous solution, also called the digestion liquor, to solubilize aluminum hydroxide from the bauxite. The supersaturated sodium aluminate solution that results from this digestion is then separated from the solid phase, which consists of the undigested residue of the ore that is rich in iron oxides and called "red mud." Next, the supersaturated sodium aluminate solution is decomposed, that is, unsaturated by precipitation of the sodium hydroxide in a starting bath before it is recharged with soda and recycled as the digestive liquor of the bauxite ore. As for the red mud, after several washings by decanting to recover the sodium aluminate liquor permeating this mud, it is usually discharged because the possibilities for developing these by-products remain limited.
The red mud consists mainly of iron, titanium and aluminum oxides and hydroxides, compounded in the form of the silico-aluminate complexes that pre-exist in the ore, but it also contains insoluble products, such as compounds of the hydroxysodalite family with a general formula: Na.sub.2 O, Al.sub.2 O.sub.3, 2SiO.sub.2, 1/3(Na.sub.2 Ca)X, mH.sub.2 O, where X may be the anion of 2Cl.sup.-, 2OH.sup.-, SO.sub.4 --or CO.sub.3 --that are formed during the sodium digestion by the combining of free soda with silica and alumina; these are also called desilication products. The soda, compounded in this way and representing at least 90% of the total soda fixed in the red mud, of course, cannot be recovered by washing in water. In addition, the quantity of the soda compound that is thus lost by being washed into the mud is all the more high as the bauxite content of silica is high.
As a rough guide, the content by weight of the main components of the dried mud can vary in the following proportions:
______________________________________ Al.sub.2 O.sub.3 10 to 20% SiO.sub.2 5 to 20% Fe.sub.2 O.sub.3 30 to 60% TiO.sub.2 5 to 10% CaO 1 to 4% Na.sub.2 O 4 to 12% ______________________________________
Loss in fire and miscellaneous: the supplement up to 100%.
It is thus frequent during the alkaline digestion of bauxites with a silica content by weight above or equal to 5% to record soda losses into the red mud on the order of 100 kg of Na.sub.2 O per ton of alumina produced. This soda, trapped in the form of sodium silico-aluminate may be 10% or even 12% by weight of the Na.sub.2 O in the dried red mud.
Many processes have been proposed to reduce the soda losses in red mud. In particular, acid processes that consist of digesting washed red mud and putting it back in suspension in water by using a sulfur compound such as SO.sub.2, H.sub.2 SO.sub.4 or carbonic acid that makes it possible to solubilize the sodium compound while the silica is kept as a solid residue, are known.
Thus FR 2372238 (U.S. Pat. No. 4,119,698) achieves sulfuric digestion of red mud at a high temperature (250 to 300.degree. C.) to dissolve most of the metallic, aluminum, iron, titanium, sodium, and calcium components in the form of sulfates that are separated from the silica by filtration. Then the titanium, iron, and aluminum are eliminated selectively so that only sodium sulfate is kept in solution. Taking into consideration the particularly severe digestion conditions that are consequently difficult to achieve, as well as numerous liquid/solid separation operations, this process has turned out to be very expensive to use and, in addition, it does not result in enhanced value of the sodium recovered in a directly recyclable form in the main Bayer process circuit.
FR 2576149 (U.S. Pat. No. 4,668,485) achieves selective digestion of the combined soda in red mud by barbotting SO.sub.2 in an aqueous solution of the mud at less than 100.degree. C. The sodium and part of the aluminum passes into solution in sulfite form. Using lime causticizing, the sulfite anion is precipitated out as calcium sulfite, and the sodium and aluminum are kept in solution as sodium aluminate that is reusable directly in the Bayer circuit. If this process makes it possible to enhance the value of the sodium extract of red mud in a better way, it also remains very difficult to use because of the problems posed by the separation of silica, which in SO.sub.2 digestion at low temperature can pass partially into a colloidal state, and by the reprocessing of calcium bisulfate, a by-product of caustification.
Processes for the basic treatment of red mud that consist of direct caustification of the mud by lime are also known. Under certain temperature and lime concentration conditions, the sodium in the desilication products can be replaced by calcium.
That is what U.S. Pat. No. 4,486,393 achieves by putting the mud in suspension in its last washing water, which also contains free soda at a temperature around 100.degree. C. and in contact with a sufficient amount of lime meant, on one hand, to substitute for the sodium compounds of the desilication products and free the soda, but also to extract part of the compounded alumina from these desilication products. This easy-to-use process, however, makes it possible to extract only 30 to 40% of the compounded soda. By performing the same operations at a much higher temperature (about 300.degree. C.), U.S. Pat. No. 4,483,830 recovers more than 90% of the compounded sodium in red mud with, on the other hand, the disadvantage of having to use processing methods that are very expensive not only in terms of investment but also in operating costs, especially, since they involve high pressure-sealed batteries.